Passive broadband long wave and mid-wave infrared optical limiter device

ABSTRACT

Method for limiting amount of radiation impinging on a radiation-sensitive detector device by directing radiation toward the detector, permitting the radiation to impinge upon the detector device when the radiation is below a predetermined threshold, and utilizing radiation having wavelengths different from signals of interest to initiate limiting of the radiation impinging upon the detector when the predetermined threshold is exceeded. The optical limiter includes an IR limiting layer pair selected so that energy from visible and near infrared radiation activates the optical limiter. The limiting layer pair may includes a layer closer to the source of radiation of e.g. vanadium dioxide, vanadium sesquioxide, or germanium crystal and a layer further from the source of radiation of e.g. chalcogenide glass, germanium crystal, or sodium chloride crystal.

FIELD OF THE INVENTION

This invention provides a passive infrared (IR) optical limiter devicefor long wave and mid-wave infrared radiation detector protection. Withrespect to infrared wavelengths, wavelengths of approximately 7 to 14microns are considered to be in the long wave infrared radiation (LWIR)region and wavelengths of approximately 3 to 5 microns are considered tobe in the mid-IR region. This invention provides an optical limiter fordetectors in the LWIR region. The optical limiter of this invention canalso be used in the mid-IR region. In a preferred embodiment, theoptical limiter device of this invention comprises an IR limiting layerpair comprising vanadium oxide and a suitable substrate, for instancegermanium or AMTIR or sodium carbonate.

BACKGROUND OF THE INVENTION

Optical limiters are used for the protection—for instance of eyes, ofphotodetectors, or of cameras—against unexpected strong illumination.The present invention provides an optical power limiting device,primarily for protection in the LWIR region. The optical limiter of thisinvention is transparent under a low level of illumination at thewavelength of interest, but is “dark” under strong incident light power.For incident power that exceeds the power threshold of the presentoptical limiter, the optical signal power transmitted through theoptical limiter is substantially constant (at the threshold level), nomatter what the incident power is. Optical limiters of the presentinvention have low initiating thresholds and broad spectral ranges.

Optical limiting devices have been made with Reverse Saturable Absorber(RSA) solutions and multi-photon absorber dyes. Such materials absorbmore light as the intensity of the incident light increases. Adisadvantage of such materials arises from their use of nonlinearoptical absorption processes. This leads to a high threshold for theoptical limiting behavior to switch on. More importantly, unlike thepresent invention, conventional RSA or multi-photon solutions cannot beused in the LWIR region.

The use of vanadium oxide interference mirrors for optical limiting inthe LWIR region has been proposed. See Konovalova et al., “Interferencesystems of controllable mirrors based on vanadium dioxide for thespectral range 0.6-10.6 μm”, J. Opt. Technol., 66 (5):391-398 (1999).The device proposed by Konovalova et al. would use vanadium oxide filmto absorb incident laser energy and change phase. A primary function ofthe Konovalova limiter is laser hardening. It is therefore a relativelynarrowband device that is not suitable for handling a broadband range ofwavelengths. The Konovalova approach requires an initiating threshold onthe order of 1 MW/cm², which is too high for many applications. A mid-IRand LWIR range limiter, for instance, must limit a continuous wavehaving a broad band but having relatively low peak power radiation(about 1 W/cm²).

It is well known that vanadium dioxide (VO₂) thin films experience phasechange from semiconductor to metal at around 68° C. Since this phasechange is solid to solid, the phase change speed can be as fast as lessthan 150 femtoseconds. In the phase change process, the refractive indexof the VO₂ varies dramatically. For example, at a wavelength of 10.6microns, the refractive index of VO₂ varies from 2.55-0.08i in thesemiconductor phase to 8-9i in the metal phase. At the differentrefractive indexes, the electromagnetic radiation is transmitted throughor reflected by the film.

FIG. 1A, taken from Verleur et al., Physics Review, 172(3):788-798(1968), depicts the temperature dependence of optical transmission at0.31 eV and resistivity of a 1000 Å film of VO₂. FIG. 1A shows that thetransmission through the film can vary from 90% to 10% as temperatureincreases from 60° C. to 70° C.

As illustrated in FIG. 1B, taken from Barker et al., Physics ReviewLetters, 17(26):1286-1289, the variation of refractive index ortransmission through the film is broadband, from near-IR to mid-IR toLWIR. In FIG. 1B, the solid curve is a fit for T<T_(t) using eightphonon modes and one band-structure mode. The crosses and squares showthe data above T_(t). The squares are low because of sample cracking.However, they illustrate the monotonic rise expected of free carrierreflection. The triangle points were taken upon cooling. Because ofthermal hysteresis, T_(t)≈63° C. for this run. The present inventionmakes use of this variable characteristic of the film to provide abroadband optical limiting device.

An object of the present invention is to protect detector devices suchas LWIR cameras against damage from continuous wave broadband radiation,such as sunlight. Because of its high temperature and brightness,continuous wave radiation can be more harmful to a detector such as acamera than a pulsed narrow bandwidth laser would be. On the other hand,because of its high temperature, the continuous wave radiation hashundreds of times more energy in the visible and near infrared spectrumthan in the long wave infrared region. The present invention makes useof the energy from visible and near infrared radiation to heat andtrigger the optical limiting function of the present optical limiterdevices.

SUMMARY OF THE INVENTION

In one embodiment, the present invention provides a passive broadbandlong wave infrared radiation (LWIR) optical limiter device made with anIR limiting layer pair selected so that energy from visible and nearinfrared radiation activates the LWIR optical limiter. The presentinvention also contemplates a camera protected against sunlight by theincorporation into the camera of a passive broadband LWIR opticallimiter device as described herein. The IR limiting layer pair of thisinvention preferably includes an external layer closer to the source ofradiation comprises a material—for example, vanadium dioxide, vanadiumsesquioxide, or germanium crystal—that can change quickly from asemiconductor phase to a metal phase upon heating and an internal layerfurther from the source of radiation comprises material—for example,chalcogenide glass, germanium crystal, or sodium chloride crystal—thatstrongly absorbs radiation except for radiation of the wavelength regionto which the detector is sensitive.

This invention also provides a method for limiting the amount ofradiation impinging on a radiation-sensitive detector device that isresponsive to signals of interest having infrared wavelengths ofapproximately 3 to 14 microns. Such a radiation-sensitive detectordevice may be, for instance, an infrared camera. This method includes:directing radiation toward the radiation-sensitive detector device, forinstance by exposing it to a source of infrared radiation and to ambientsunlight; permitting the radiation to impinge upon, for instance, aninfrared focal plane array within the radiation-sensitive detectordevice when the radiation is below a predetermined threshold (e.g., whenthe heat generated by the infrared radiation and the ambient sunlight isno greater than 1 W/cm²); and utilizing radiation having wavelengthsdifferent from the signals of interest to initiate limiting of theradiation impinging upon the radiation-sensitive detector device whenthe predetermined threshold is exceeded. More specifically, this laststep may, for instance, utilize heat generated by the radiation havingwavelengths different from the signals of interest to change the stateof a material in the path of the infrared radiation from a semiconductorphase to a metal phase, thereby preventing impingement of the infraredradiation on the infrared focal plane array in the camera.

In a variant method of this invention, the signals of interest haveinfrared wavelengths of approximately 7 to 14 microns, and the radiationused to initiate limiting of the radiation impinging upon theradiation-sensitive detector device includes radiation havingwavelengths in the range 3 to 5 microns.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be more fully understood from the detaileddescription given hereinafter and from the accompanying drawings. Thedrawings are not to scale, and are presented for illustrative purposesonly. Thus the drawings are not intended to limit the present invention.

FIG. 1A depicts the temperature dependence of optical transmission in aVO₂ film.

FIG. 1B shows the variation of refractive index or transmission througha VO₂ film.

FIG. 2A is a schematic cross-sectional illustration of a two layerdevice of the present invention. FIG. 2B is a schematic cross-sectionalillustration of a multi-layer layer device of the present invention.

FIG. 3 is a schematic cross-sectional illustration of an embodiment ofthe present invention in a camera.

DETAILED DESCRIPTION OF THE INVENTION

This invention provides a passive IR optical limiter device for longwave infrared radiation (LWIR) detector protection. The LWIR opticallimiter of the present invention makes use of a phase change in a VO₂film at around 68° C., at which temperature the VO₂ changes from asemiconductor phase to a metal phase. While the semiconductor phase isessentially transparent, the metal phase is essentially opaque. Amaterial—which strongly absorbs all optical radiation except in thewavelength region that the detector is interested in—is provided as athin substrate layer attached to the VO₂ thin film. When the intensityof the incident light illumination is over a certain threshold, thetemperature of the VO₂ film increases to over the 68° C. phase changetemperature, and the VO₂ film changes from “transparent” to “dark”.

Since the attached layer or substrate absorbs radiation, the LWIRoptical limiter of this invention is designed for relatively low peakpower, continuous wave, broad bandwidth light fluence. An importantfeature of the LWIR limiter device of the present invention is that itis broadband, covering the whole LWIR region. The attached layer whichmakes up part of the present device strongly absorbs solar radiation, sothat the optical limiter's temperature can increase quickly and respondquickly to a variation in the solar radiation. The LWIR optical limiterof this invention is a true passive device, but nonetheless it providesa quick response. Being composed principally of two thin layers, it issmall, compact, and light.

The LWIR optical limiter of this invention is designed for use in anenvironment subject to continuous wave incident light (e.g., sunlight).However, the optical limiter of this invention can also be used in laserhardening. The VO₂ film has a very small absorption in the LWIR region,and this absorption by the VO₂ film increases with increasingtemperature. A high intensity (˜1 MW/cm²) incident laser pulse canincrease the film temperature dramatically, so that the film reaches thephase change temperature in, e.g., 10 to 30 nanoseconds. The laserenergy is then either absorbed or reflected by the VO₂ film before itreaches the detector.

The LWIR limiting device of the present invention may comprise a VO₂film having a thickness of 10-1000 nm, preferably about 100 nm. Attachedto the VO₂ film is a thin layer of a substrate material that stronglyabsorbs any radiation except the wavelength region to which the detectoris sensitive. Germanium crystal is one such material. Various salts,e.g. sodium chloride, may also be used as the substrate material. Sodiumchloride crystal has a low refractive index and is transparent toradiation having wavelengths of from 0.2 to 15 microns. A preferredmaterial for this component of the optical limiter device of the presentinvention is AMTIR. The name AMTIR is an acronym for amorphous materialtransmitting infrared radiation. More specifically, AMTIR is achalcogenide (As_(x)Ge_(y)Se_(z)) type glass-like amorphous materialwith high homogeneity that is able to transmit in the infrared. AMTIR1,for instance, is used for infrared windows, lenses, and prisms whentransmission in the range of 1-14microns is desired. AMTIR1s compositionof Ge₃₃As₁₂Se₅₅ makes it somewhat similar to germanium in its mechanicaland optical properties. AMTIR1performs especially well in the 8- 12micron spectral region where its absorption and dispersion are thelowest. AMTIR1 materials are available commercially, for instance fromArgus International, Ltd., of Scotts Valley, Calif. The VO₂film-containing LWIR limiting device of the present invention mentionedabove may comprise a Ge₃₃As₁₂Se₅₅ about 0.1 to 5 mm in thickness. Thetwo-layer device of the present invention is illustrated schematicallyin FIG. 2A.

The two-layer device described above is the basic embodiment of thepresent invention. However, optical limiters in accordance with thisinvention may contain additional layers, as illustrated schematically inFIG. 2B, For instance a broadband interference filter coating may bepositioned on the surface of the substrate layer away from the VO₂layer. The broadband interference filter coating transmits radiationsignal which has wavelength in the region of 7˜14 microns but reflectsthe radiation of all the other wavelengths. One example of such acoating is made from thorium fluoride and zinc selenide. The thoriumfluoride is coated directly onto the prism, and it is covered by a layerof zinc selenide. Both the ThF₄ layer and the ZnSe layer are about 10 to1000 nanometers in thickness. Such coatings provide transmittance of upto 98.5% throughout the 7-14 micron wavelength range. Another optionalcoating layer that may be used in connection with the present inventionis a protective coating over the VO₂ layer.

The detection sensitive wavelength range for a detector in accordancewith the present invention is 8 to 12 microns for cooled detector or 7to 14 microns for uncooled detector. The VO₂ film employed in thisinvention is so thin that it does not need much heat to increase itstemperature. At room temperature, the VO₂ is in its semiconductor phaseand has a low refractive index, so that it absorbs very littleradiation. The attached layer, as indicated above, does not absorb theradiation to which the detector is sensitive. The optical limiter ofthis invention is thus “transparent” to incident low-intensity light.However, continuous wave harmful “bright” illumination, such assunlight, usually has hundreds of times more energy in the visible andnear IR region than in the LWIR region. In accordance with thisinvention, the vanadium dioxide film and its substrate strongly absorbthe radiation in the visible and near IR region. The radiation isconverted to heat which heats the VO₂ film. When the environmentalradiation is stronger than a certain threshold, the VO₂ film temperatureincreases above the 68° C. phase change temperature, at which point theVO₂ film changes to the metal phase and starts to reflect the radiation,including the wavelength to which the detector is sensitive—that is,LWIR. Thus the optical limiter of this invention is “dark” at highillumination fluence.

The present optical limiter device may also include a heat loss deviceto balance the temperature. Accordingly, when the incident light fluencedecreases below the threshold, the heat-loss device may be employed toblow the heat away from the vanadium dioxide film, at which point thefilm recovers back to the semiconductor phase.

The description of the present invention in this application is focusedon VO₂. However, other vanadium oxides (such as vanadium sesquioxide,V₂O₃) and some non-vanadium semiconductor materials, e.g., germaniumcrystal, also can change from a semiconductor phase to a metal phaseupon heating. In their semiconductor phases, these materials aretransparent to LWIR radiation. In their metal phases, these materialsreflect LWIR radiation. The phase change is temperature-dependent. Forexample, the phase change temperature of V₂O₃ is −123° C. and the phasechange temperature of germanium crystal is 130° C. Inasmuch as the phasechange temperature of VO₂ is just above room temperature (viz., 68° C.),VO₂ is particularly convenient for many applications. Those skilled inthe art are capable of selecting a particular phase-changing materialfor use depending upon the environment in which it is to be employed.Those skilled in the art are also capable of adjusting the phase changetemperature of such phase-changing materials. For instance, the 68° C.phase change temperature of VO₂ can be lowered by doping titanium onto alayer of VO₂.

FIG. 3 is a schematical cross-sectional view of an embodiment of theLWIR optical limiter device of the present invention, located within acamera. As depicted in FIG. 3, the LWIR optical limiter, comprised ofVO₂ film and substrate, is located in front of the Infrared Focal PlaneArray (IRFPA). Radiation, e.g. sunlight, passes through the lens intothe camera. Near infrared radiation in the sunlight heats the VO₂ film.At about 100 W/cm² near infrared incidence, the VO₂ switching time (thatis, phase change time) is on the order of milliseconds.

The vanadium dioxide layer can be incorporated into the device of thepresent invention by a two-step procedure: pulsed laser deposition (PLD)of a substoichiometric vanadium oxide, followed by annealing to createVO₂. The vanadium dioxide layer is deposited on the substrate. Thepulsed laser deposition is carried out in a commercial PLD chamber (forinstance, an Epion PLD 3000). In a typical PLD procedure, the beam froma KrF excimer laser at a wavelength of 248 nm with 25 Hz pulse rate isfocused onto a pure vanadium target at a fluence of approximately 4mJ/cm². The beam energy is controlled by splitting off a very smallfraction of the laser beam prior to entering the PLD chamber and usingit for feedback through the control of the PLD system. The number oflaser pulses required to deposit 100 nm of oxide on the substrate istypically on the order of 5×10⁴. Laser rastering and the distancebetween the rotating target and the substrate are adjusted so that theablation plume covers the substrate uniformly. A typicaltarget-substrate distance is 7 cm. This procedure is conducted at roomtemperature. The background vacuum level before introducing oxygen ismaintained under 3×10⁻⁶ Torr. The PLD-deposited film is subsequentlyannealed. A typical oxygen pressure in the deposition and annealingprocesses is 5 mTorr.

In addition to the uses described herein, the optical limiter of thisinvention can be incorporated into a variety of other usefulapparatuses. For instance, the optical limiter of this invention can beincorporated into the passive broadband long wave and mid-wave infraredoptical limiting prism that is disclosed in copending U.S. patentapplication Ser NO. 11/012,106 by Wu, Dalakos, Lawrence, and Lorraine,entitled “Passive Broadband Long Wave and Mid-Wave Infrared OpticalLimiting Prism”, filed concurrently herewith. The entire disclosure ofthis copending application is expressly incorporated by reference in thepresent application. Those skilled in the art will readily conceive ofstill other apparatus configurations in which the optical limiter ofthis invention can be used.

The present invention is described as a broadband infrared limiter. Itwill be understood, however, that the description provided hereinaboveis merely illustrative of the application of the principles of thepresent invention, the scope of which is to be determined by the claimsviewed in light of the specification. Other variants and modificationsof the present invention will be readily apparent to those skilled inthe art.

1. A passive broadband long wave infrared radiation (LWIR) opticallimiter device comprising an infrared (IR) limiting layer pair selectedso that energy from visible and near infrared radiation havingwavelengths in the range 3 to 5 microns activates said LWIR opticallimiter and thereby prevents undesired radiation from impinging upon aradiation-sensitive detector device protected by said LWIR opticallimiter device, wherein said optical limiter device comprises an IRlimiting layer pair in which an external layer closer to a source ofradiation comprises a material, selected from the group consisting ofvanadium dioxide and vanadium sesquioxide, that can change from asemiconductor phase to a metal phase upon heating and in which aninternal layer further from the source of radiation compriseschalcogenide glass material that strongly absorbs radiation except forradiation of the wavelength region to which the device is sensitive,wherein the external layer and the internal layer are adjacent to eachother, and wherein said device comprises a broadband interference filtercoating that transmits radiation of 7˜4 microns wavelength and reflectsradiation of other wavelengths.
 2. A passive broadband long waveinfrared radiation (LWIR) optical limiter device comprising an infrared(IR) limiting layer pair selected so that energy from visible and nearinfrared radiation activates said LWIR optical limiter, wherein saiddevice comprises an IR limiting layer pair in which an external layercloser to a source of radiation comprises a layer of vanadium dioxideabout 10 to 1000 nm in thickness, that can change from a semiconductorphase to a metal phase upon heating and in which an internal layerfurther from the source of radiation comprises a layer of Ge₃₃As₁₂Se₅₅about 0.1 to 5 mm in thickness that strongly absorbs radiation exceptfor radiation of the wavelength region to which the device is sensitive.3. The optical limiter device of claim 2, wherein said device comprisesa broadband interference filter coating that transmits radiation of 7-14microns wavelength and reflects radiation of other wavelengths.
 4. Acamera protected against sunlight by the incorporation into said cameraof the passive broadband LWIR optical limiter device of claim
 3. 5. Theoptical limiter device of claim 3, wherein said broadband interferencefilter coating comprises layers of thorium fluoride and zinc selenideabout 10 to 1000 nanometers in thickness.
 6. A camera protected againstsunlight by the incorporation into said camera of the passive broadbandLWIR optical limiter device of claim
 5. 7. A camera protected againstsunlight by the incorporation into said camera of the passive broadbandLWIR optical limiter device of claim
 2. 8. A camera protected againstsunlight by the incorporation into said camera of a passive broadbandLWIR optical limiter device comprising an infrared (IR) limiting layerpair selected so that energy from visible and near infrared radiationactivates said LWIR optical limiter, wherein said camera comprises aninfrared focal plane array, and said device incorporated into saidcamera comprises an IR limiting layer pair in which an external layercloser to a source of radiation comprises a material, selected from thegroup consisting of vanadium dioxide and vanadium sesquioxide, that canchange from a semiconductor phase to a metal phase upon heating and inwhich an internal layer further from the source of radiation compriseschalcogenide glass material that strongly absorbs radiation except forradiation of the wavelength region to which the device is sensitive,wherein the external layer and the internal layer are adjacent to eachother.